Pyrochlore solution

Voigt, J. A. Tuttle, B. A. Headley, T. J. Lamppa, D. L. 1995. The pyrochlore-to-perovskite transformation in solution-derived lead zirconate titanate thin films. In Ferroelectric Thin Films IV, edited by Tuttle, B. A. Desu, S. B. Ramesh, R. Shiosaki,T. Mat. Res. Soc. Symp. Proc. 361 395 102. 

Literature on flotation of gold, PGMs, rare earths and various oxides is rather limited, compared to literature on treatment of sulphide-bearing ores. As mentioned earlier, the main problem arises from the presence of gangue minerals in the ore, which have flotation properties similar to those of valuable minerals. These minerals have a greater floatability than that of pyrochlore or columbite. In the beneficiation of oxide minerals, finding a selectivity solution is a major task. 

The bixbyite structure of e.g. SC2O3 represents a solution to the problem of filling three quarters of the tetrahedral sites of cubic close-packed Sc with O. It is not the simplest solution that would be the structure obtained by filling the tetrahedral sites not filled in CU2O (i.e. the pyrochlore structure of AgSb03, with Ag, Sb replaced by Sc, cf. Sect. 2.1.1.3). With normal Sc-O bond lengths that structure would have short O... O distances, and so an alternative structure that allows adjustment of the O... O and M... M distances is adopted. 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Ca,Gd,Ce,Hf)2(Ti,Mo)207 in neutral to basic solutions. Near steady-state Mo concentrations (0-4 ppb range) yield apparent dissolution rates of 2-6 x 10 3 g/m2/d for both samples at pH = 6-8 and flow rate = 2 mL/d. In total, the results for (Ca,Gd,Ce,Hf)2(Ti,Mo)207 indicate a weak pH dependence with a minimum at pH = 7. Similar pyrochlore-rich ceramics were studied by Hart et al. (2000), who showed that the release rate of Pu dropped from approximately 10 3 g/m2/d to 10 5 g/m2/d or less after nearly one year in pure water at 90 °C (Fig. 7). The release rates of U and Gd in these experiments were higher than Pu by factors of about 10 and 100, respectively. 

Another oxide family involving zirconium oxide has been examined as a possible alternative to YSZ as electrolytes in SOFCs. They are the pyrochlore solid solutions of the general formula M2Zr207, where M is a rare-earth metal [25-29]. Zirconium can be substituted by a transition metal such as titanium. Moon et al. [29] have in-... 

Ample studies on pyrochlore oxide electrolytes have been carried out, particularly on Gd2Ti207- and Gd2Zr207-based conductors, where the Gd2Ti. Zrx07 solid solution is of great interest because the x=0 member is an ionic insulator whereas the x = 1 end member is a good oxide ion conductor [96,97],... 

Oxygen (O2-) anion conductors - stabilized zir-conia, stabilized - bismuth oxide, - BIMEVOX, doped cerium dioxide, numerous perovskite-type - solid solutions derived from Ln(A)B (B") 03 (A = Ca, Sr, Ba B = Ga, Al, In B" = Mg, Ni, Co, Fe), La2Mo207 and its derivatives, pyrochlores based on Ln2Ti07. 

Doping of CrF, with Fe(III) or Mg(II), would be expected to result in the formation of solid solutions. However, depending on the Fe(III) concentration, there are two distinct regions, a concentration range from 0 to 41% Fe(III) with the pyrochlore structure and a second range from 65 to 100% Fe with the HTB structure [93], The surface areas of the latter samples are about double those of the former. At about 65% Fe(III) a maximum in the acidity of the samples was observed, acidity decreases with further increase of Fe(III). These phases however, exhibit only average catalytic activity. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as tf-type Fe2 Ty03 and type Ni1 LyO. Thick film resistors are also made from transition-metal oxide solid solutions. Glass-bonded By 2 Pb2yRu207 having the pyrochlore [12174-36-6] structure is typical. 

The cubic pyrochlore-based ceria-zirconia solid solution is a good material. However, it has been reported that the reduction temperature has become higher after the high-temperature oxidation.This is due to the transformation from pyrochlore-based-cubic to r -tetragonal phase which is more stable than it, and r ... 

Recently one of the solutions to overcome this problem has been proposed.This does concern surface modification of the pyrochlore-based oxide.s. It is known that cerium and zirconium chlorides provide vapor phase complexes with aluminum chloride at elevated temperatures.The new surface modification technique utilizes the formation of these vapor complexes to remove and modify the top surface of the pyrochlore ceria-zirconia solid solution. This method is named "chemical filing". Application of the above complexes formation has already been demonstrated for the vapor phase extraction and mutual separation of rare earths based on the so-called chemical vapor transport (CVT). ... 

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